1. Field of the Invention:
The present invention relates to the field of position measuring systems, and particularly to such systems employing digital-to-analog converters for accepting digital inputs and responsively providing analog signals to position-measuring devices such as Inductosyn transducers, for position control and position readout applications.
2. Description of the Prior Art:
One such prior art converter is described in Tripp U.S. Pat. No. 3,686,487. In that patent a digital sine/cosine generator is disclosed in which a clock signal is counted down through parallel first and second counters. A generation means is provided for accepting a digital input of n bits corresponding to an error signal generator by the transducer representative of a change in the relative position of two members of an Inductosyn position measuring transducer and responsively generating a difference in count between the two counters equal to the digital input, so as to relatively phase shift the outputs of the two counters. The relatively phase-shifted counter outputs are then logically combined to form one or more pulse-width modulated rectangular wave signals which are used to excite the windings of the transducer. In that converter the first and second counters have a count range of N, so that for a digital of n bits each of the pulse-width modulated signals includes a fundamental frequency component having an amplitude proportional to a trigonometric function (e.g. sine or cosine) of an angle .theta., where .theta. equals (n/N)360.degree..
The above-noted converter is typically used to divide the periodic measurement cycle of an Inductosyn transducer into N parts. For example, for a typical Inductosyn transducer cycle of 0.2 inch (5.08 mm.), and for first and second counters having a count range of 2000, the 0.2 inch cycle is divided into 2000 parts, i.e., each digital bit of the count range represents 1.times.10.sup.-.sup.4 inch (2.54.times.10.sup.-.sup.3 mm.).
In such position measurement systems two basic configurations are generally employed. In the first configuration the error signal is detected as being in one of two error states (positive or negative) with no intermediate dead zone in which the error signal may vary with no corresponding change in converter output. Such a system requires constant correction, and as a result stabilization is difficult. The second common configuration uses a three-state error signal which is detected as being positive, zero, or negative. In the intermediate zero state no corrections to the system are made. As a consequence, this type of system is more easily stabilized, but the stabilization is a function of overall system gain. For example, if the system gain is very high the predetermined magnitude of the error signal zero state becomes relatively insignificant, and in effect a two-state error signal configuration is attained. However, if the system gain is very low, the intermediate error signal zero state becomes relatively large, and large positional errors must occur before the positive or negative states ar detected and positional correction in stituted.